This invention relates to a catalytic technique for cracking heavy petroleum stocks and upgrading light olefin gas to valuable olefinic hydrocarbons. In particular, it provides a continuous integrated process for isomerizing and oligomerizing olefinic light gas byproduct of FCC cracking to produce C.sub.4 .sup.+ hydrocarbons, such as iso-olefin, olefinic gasoline or high quality distillate. Ethene, propene and/or butene containing gases, byproducts of petroleum cracking in a fluidized catalytic cracking (FCC) unit, may be upgraded by contact with a crystalline medium pore siliceous zeolite catalyst.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.4 -C.sub.5 tertiary olefins, gasoline, etc. In addition to basic chemical reactions promoted by zeolite catalysts having a ZSM-5 structure, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.4 alkenes.
Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as chemical feedstocks and liquid fuels, especially the C.sub.4 .sup.+ aliphatic hydrocarbons. Product distribution for hydrocarbons can be varied by controlling process conditions, such as temperature, pressure, catalyst activity and space velocity.
At low pressure and moderately high temperature, thermodynamics restrict the olefin distribution to relatively low molecular weight. This is the basis for the olefin interconversion process, i.e., to operate under conditions where lower olefins, such as C.sub.2 -C.sub.4 olefins can be converted to an equilibrium distribution of olefins with iso-butenes and iso-pentenes maximized. The Mobil Olefin Interconversion ("MOI") process as utilized in the present invention can use fixed bed, moving bed or fluid bed reactors containing zeolite type catalysts such as ZSM-5. Operating conditions encompass temperatures between 250.degree. and 550.degree. C. and low pressures, generally between 100 and 1500 kPa. Gasoline (C.sub.5 -C.sub.10) is also formed at elevated temperature (e.g., up to about 400.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 250 to 2900 kPa. Olefinic gasoline can be produced and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products.
Recently it has been found that olefinic light gas can be upgraded to hydrocarbons rich in iso-olefins by catalytic conversion in a fluidized bed of solid medium pore acid zeolite catalyst under effective reaction severity conditions. Such a fluidized bed operation typically requires oxidative regeneration of coked catalyst to restore zeolite acidity for further use, while withdrawing spent catalyst and adding fresh acid zeolite to maintain the desired average catalyst activity in the bed. This technique is particularly useful for upgrading FCC light gas, which usually contains significant amounts of ethene, propene, C.sub.1 -C.sub.4 paraffins and hydrogen produced in cracking heavy petroleum oils or the like. Furthermore, it has been found that C.sub.6 + olefinic components can be interconverted to iso-olefin rich C.sub.5 - over medium pore zeolites. Therefore, C.sub.6 + olefins made during the course of light olefin interconversion can be recycled to the same reactor to produce additional C.sub.4 -C.sub.5 iso-olefins.
Alternatively, the C.sub.6 + olefins can be more selectively upgraded in another interconversion reactor operating under conditions to maximize C.sub.4 -C.sub.5 iso-olefins yield.
Economic benefits and increased product quality can be achieved by integrating the FCC and olefin interconversion units in a novel manner. It is the primary object of this invention to eliminate the olefins upgrading catalyst regeneration system which results in significant process investment saving and improved process safety. Another object of this invention is to eliminate the olefins upgrading spent catalyst stripper which results in significant process investment/operating cost saving. Another object of the present invention is to further extend the usefulness of the medium pore acid zeolite catalyst used in the olefinic light gas upgrading reaction by withdrawing a portion of partially deactivated and coked zeolite catalyst and admixing the withdrawn portion with cracking catalyst in a primary FCC reactor stage. Prior efforts to increase the octane rating of FCC gasoline by addition of zeolites having a ZSM-5 structure to large pore cracking catalysts have resulted in a small decrease in gasoline yield, increase in gasoline quality, and increase in light olefin byproduct.